The present invention relates to devices for restricting or controlling the movement or loading levels on joints in the human or animal body.
The human or animal body uses articular cartilage to surface many of its joints. This tissue tolerates relatively high levels of compression while having a low coefficient of friction—approximately that of wet ice on wet ice.
Bone, on which the cartilage is supported, is stiffer and stronger. Away from the joints, bone normally forms in large, thick-walled tubes. However, under the cartilage at the joints, the bone forms a three dimensional mesh of so called “cancellous” bone. Cancellous bone is more compliant than the rest of the bone structure and helps spread the load that the cartilage experiences, thus reducing the peak stresses on the cartilage.
Both cartilage and bone are living tissues that respond and adapt to the loads they experience. There is strong evidence that the loads that joint surfaces experience can be categorised into four regions or “loading zones”.
1. Under-Loading Zone.
If a joint surface remains unloaded for appreciable periods of time the cartilage tends to soften and weaken.
2. Healthy Zone.
Joint surfaces can and do last a lifetime and if they experience healthy levels of load they can be considered to effectively last indefinitely.
3. Tolerant Zone.
As with engineering materials that experience structural loads, both bone and cartilage begin to show signs of failure at loads that are below their ultimate strength. Unlike engineering materials, however, cartilage and bone have some ability to repair themselves, bone more so. There are levels of loading that will cause micro-structural problems and trigger the repair processes. The body can tolerate these load levels as long as it has time to recuperate.
4. Overloaded Zone.
There comes a level of load at which the skeleton will fail catastrophically. If the load level on a joint surface reaches this level even once then there will be severe consequences.
One of the major consequences of excessive loading is osteoarthritis. This loading could be either from a single overload in the overloaded zone or from loading within the tolerant zone too frequently.
The picture of safe joint loading is further complicated by the cascade of events that occur during the onset of osteoarthritis. These events include the break up of the cartilage, and bone ‘sclerosis’ in which the bone becomes denser and stiffer. This means that the maximum level of loading that can be considered healthy or tolerated falls, almost certainly to levels below that experienced in walking and standing.
Newly implanted grafts or tissue-engineered constructs will also have lower tolerance limits while they are establishing themselves within the joint.
In fact the treatment of osteoarthritis and other conditions is severely hampered when a surgeon is not able to control and prescribe the levels of joint load. Furthermore, bone healing research has shown that some mechanical stimulation can enhance the healing response and it is likely that the optimum regime for a cartilage/bone graft or construct will involve different levels of load over time, eg. during a particular treatment schedule.
There is a need for a device that will facilitate the control of load on a joint undergoing treatment or therapy, to enable use of the joint within the healthy loading zone, or even within the healthy and tolerant loading zones, during the treatment episode.
There is further need for a device to preferably provide such control while allowing full, or relatively full mobility of a patient undergoing the treatment.
Such devices would be desirable particularly during the early treatment of, for example, an osteoarthritic joint. Under an appropriate treatment regime providing controlled loading, the condition of the joint may improve, possibly back to full health.
In the prior art, existing load controlling regimes and devices for use in treatment or therapy of articulating joints include the following.
a) Bed-rest or isolation of a joint is possible but, as indicated above, the long-term consequences of applying no load or generally maintaining the joint in the underloaded zone are not good.
b) Passive movement of a joint has been tried with some success. During this treatment, movement is applied to the joint by an external device while the joint is rested. However, this does not give the opportunity to vary the load levels on the joint, eg. to work the joint within the healthy zone for that joint at any given stage of the treatment program.
c) Traction across a joint has long been used to counteract the compressive loads normally experienced by the joint. This is done either in bed or using an external fixator. Fixators exist which not only apply traction, but also have simple hinges to allow some joint motion.
d) External braces have been used to apply a bending moment across the joint and at 90.degree. to the motion to move the centre of pressure from one part of the joint to another. However, since these braces are not attached directly to the skeleton, control of the applied loads is poor.
According to one aspect, the present invention provides an apparatus for controlling the load on articular cartilage of a human or animal joint comprising:
a first fixation assembly for attachment to a first bone;
a second fixation assembly for attachment to a second bone; and
a link assembly coupled to the first fixation assembly by a first pivot and coupled to the second fixation assembly by a second pivot,
the first and second fixation assembly thereby each being angularly displaceable relative to the link assembly.
According to another aspect, the present invention provides a method of controlling loading on a joint comprising the steps of:
attaching a first fixation assembly to a first bone;
attaching a second fixation assembly to a second bone, the second bone being connected to the second bone by an articulating joint;
coupling said first fixation assembly and said second fixation assembly by way of a link assembly so that said first fixation assembly and said second fixation assembly are each angularly displaceable relative to the link assembly.